There are two basic marine fuel types: distillate based marine fuel, also known as Marine Gas Oil (MGO) or Marine Diesel Oil (MDO); and residual based marine fuel, also known as heavy marine fuel oil (HMFO). Distillate based marine fuel both MGO and MDO, comprises petroleum middle distillate fractions separated from crude oil in a refinery via a distillation process. Gasoil (also known as medium diesel) is a petroleum middle distillate in boiling range and viscosity between kerosene (light distillate) and lubricating oil (heavy distillate) containing a mixture of C10 to C19 hydrocarbons. Gasoil (a heavy distillate) is used to heat homes and is used blending with lighter middle distillates as a fuel for heavy equipment such as cranes, bulldozers, generators, bobcats, tractors and combine harvesters. Generally maximizing middle distillate recovery from heavy distillates mixed with petroleum residues is the most economic use of these materials by refiners because they can crack gas oils into valuable gasoline and distillates in a fluid catalytic cracking (FCC) unit. Diesel oils for road use are very similar to gas oils with road use diesel containing predominantly contain a middle distillate mixture of C10 through C19 hydrocarbons, which include approximately 64% aliphatic hydrocarbons, 1-2% olefinic hydrocarbons, and 35% aromatic hydrocarbons. Distillate based marine fuels (MDO and MGO) are essentially road diesel or gas oil fractions blended with up to 15% residual process streams, and optionally up to 5% volume of polycyclic aromatic hydrocarbons (asphaltenes). The residual and asphaltene materials are blended into the middle distillate to form MDO and MGO as a way to both swell volume and productively use these low value materials.
Asphaltenes are large and complex polycyclic hydrocarbons with a propensity to form complex and waxy precipitates, especially in the presence of aliphatic (paraffinic) hydrocarbons that are the primary component of Marine Diesel. Once asphaltenes have precipitated out, they are notoriously difficult to re-dissolve and are described as fuel tank sludge in the marine shipping industry and marine bunker fueling industry. One of skill in the art will appreciate that mixing Marine Diesel with asphaltenes and process residues is limited by the compatibility of the materials and formation of asphaltene precipitates and the minimum Cetane number required for such fuels.
Residual based fuels or Heavy Marine Fuel Oil (HMFO) are used by large ocean-going ships as fuel for large two stroke diesel engines for over 50 years. HMFO is a blend of the residues generated throughout the crude oil refinery process. Typical refinery streams combined to from HMFO may include, but are not limited to: atmospheric tower bottoms (i.e. atmospheric residues), vacuum tower bottoms (i.e. vacuum residues) visbreaker residue, FCC Light Cycle Oil (LCO), FCC Heavy Cycle Oil (HCO) also known as FCC bottoms, FCC Slurry Oil, heavy gas oils and delayed cracker oil (DCO), deasphalted oils (DAO); heavy aromatic residues and mixtures of polycylic aromatic hydrocarbons, reclaimed land transport motor oils; pyrolysis oils and tars; aspahltene solids and tars; and minor portions (often less than 20% vol.) of middle distillate materials such as cutter oil, kerosene or diesel to achieve a desired viscosity. HMFO has a higher aromatic content (especially polynuclear aromatics and asphaltenes) than the marine distillate fuels noted above. The HMFO component mixture varies widely depending upon the crude slate (i.e. source of crude oil) processed by a refinery and the processes utilized within that refinery to extract the most value out of a barrel of crude oil. The HMFO is generally characterized as being highly viscous, high in sulfur and metal content (up to 5 wt %), and high in asphaltenes making HMFO the one product of the refining process that has historically had a per barrel value less than feedstock crude oil.
Industry statistics indicate that about 90% of the HMFO sold contains 3.5 weight % sulfur. With an estimated total worldwide consumption of HMFO of approximately 300 million tons per year, the annual production of sulfur dioxide by the shipping industry is estimated to be over 21 million tons per year. Emissions from HMFO burning in ships contribute significantly to both global marine air pollution and local marine air pollution levels.
The International Convention for the Prevention of Pollution from Ships, also known as the MARPOL convention or just MARPOL, as administered by the International Maritime Organization (IMO) was enacted to prevent marine pollution (i.e. marpol) from ships. In 1997, a new annex was added to the MARPOL convention; the Regulations for the Prevention of Air Pollution from Ships—Annex VI to minimize airborne emissions from ships (SOx, NOx, ODS, VOC) and their contribution to global air pollution. A revised Annex VI with tightened emissions limits was adopted in October 2008 and effective 1 Jul. 2010 (hereafter called Annex VI (revised) or simply Annex VI).
MARPOL Annex VI (revised) adopted in 2008 established a set of stringent air emissions limits for all vessel and designated Emission Control Areas (ECAs). The ECAs under MARPOL Annex VI are: i) Baltic Sea area—as defined in Annex I of MARPOL—SOx only; ii) North Sea area—as defined in Annex V of MARPOL—SOx only; iii) North American—as defined in Appendix VII of Annex VI of MARPOL—SON, NON and PM; and, iv) United States Caribbean Sea area—as defined in Appendix VII of Annex VI of MARPOL—SON, NON and PM.
Annex VI (revised) was codified in the United States by the Act to Prevent Pollution from Ships (APPS). Under the authority of APPS, the U.S. Environmental Protection Agency (the EPA), in consultation with the United States Coast Guard (USCG), promulgated regulations which incorporate by reference the full text of Annex VI. See 40 C.F.R. § 1043.100(a)(1). On Aug. 1, 2012 the maximum sulfur content of all marine fuel oils used onboard ships operating in US waters/ECA was reduced from 3.5% wt. to 1.00% wt. (10,000 ppm) and on Jan. 1, 2015 the maximum sulfur content of all marine fuel oils used in the North American ECA was lowered to 0.10% wt. (1,000 ppm). At the time of implementation, the United States government indicated that vessel operators must vigorously prepare to comply with the 0.10% wt. (1,000 ppm) US ECA marine fuel oil sulfur standard. To encourage compliance, the EPA and USCG refused to consider the cost of compliant low sulfur fuel oil to be a valid basis for claiming that compliant fuel oil was not available for purchase. For over five years there has been a very strong economic incentive to meet the marine industry demands for low sulfur HMFO, however technically viable solutions have not been realized and a premium price has been commanded by refiners to supply a low sulfur HMFO compliant with Annex VI sulfur emissions requirements in the ECA areas.
Since enactment in 2010, the global sulfur cap for HMFO outside of the ECA areas was set by Annex VI at 3.50% wt. effective 1 Jan. 2012; with a further reduction to 0.50% wt, effective 1 Jan. 2020. The global cap on sulfur content in HMFO has been the subject of much discussion in both the marine shipping and marine fuel bunkering industry. There has been and continues to be a very strong economic incentive to meet the international marine industry demands for low sulfur HMFO (i.e. HMFO with a sulfur content less than 0.50 wt. %. Notwithstanding this global demand, solutions for transforming high sulfur HMFO into low sulfur HMFO have not been realized or brought to market. There is an on-going and urgent demand for processes and methods for making a low sulfur HMFO compliant with MARPOL Annex VI emissions requirements.
Replacement of Heavy Marine Fuel Oil with Marine Gas Oil or Marine Diesel:
One primary solution to the demand for low sulfur HMFO to simply replace high sulfur HMFO with marine gas oil (MGO) or marine diesel (MDO). The first major difficulty is the constraint in global supply of middle distillate materials that make up 85-90% vol of MGO and MDO. It is reported that the effective spare capacity to produce MGO is less than 100 million metric tons per year resulting in an annual shortfall in marine fuel of over 200 million metric tons per year. Refiners not only lack the capacity to increase the production of MGO, but they have no economic motivation because higher value and higher margins can be obtained from using middle distillate fractions for low sulfur diesel fuel for land-based transportation systems (i.e. trucks, trains, mass transit systems, heavy construction equipment, etc.).
Blending:
Another primary solution is the blending of high sulfur HMFO with lower sulfur containing fuels such as MGO or MDO low sulfur marine diesel (0.1% wt. sulfur) to achieve a Product HMFO with a sulfur content of 0.5% wt. In a straight blending approach (based on linear blending) every 1 ton of high sulfur HSFO (3.5% sulfur) requires 7.5 tons of MGO or MDO material with 0.1% wt. S to achieve a sulfur level of 0.5% wt. HMFO. One of skill in the art of fuel blending will immediately understand that blending hurts key properties of the HMFO, specifically lubricity, fuel density, CCAI, viscosity, flash point and other important physical bulk properties. Blending a mostly paraffinic-type distillate fuel (MGO or MDO) with a HMFO having a high poly aromatic content often correlates with poor solubility of asphaltenes. A blended fuel is likely to result in the precipitation of asphaltenes and/or waxing out of highly paraffinic materials from the distillate material forming an intractable fuel tank sludge. Fuel tank sludge causes clogging of filters and separators, transfer pumps and lines, build-up of sludge in storage tanks, sticking of fuel injection pumps, and plugged fuel nozzles. Such a risk to the primary propulsion system is not acceptable for a ship in the open ocean.
It should further be noted that blending of HMFO with marine distillate products (MGO or MDO) is not economically viable. A blender will be taking a high value product (0.1% S marine gas oil (MGO) or marine diesel (MDO)) and blending it 7.5 to 1 with a low value high sulfur HMFO to create a final IMO/MARPOL compliant HMFO (i.e. 0.5% wt. S Low Sulfur Heavy Marine Fuel Oil—LSHMFO) which will sell at a discount to the value of the principle ingredient (i.e. MGO or MDO).
Processing of Residual Oils.
For the past several decades, the focus of refining industry research efforts related to the processing of heavy oils (crude oils, distressed oils, or residual oils) has been on upgrading the properties of these low value refinery process oils to create middle distillate and lighter oils with greater value. The challenge has been that crude oil, distressed oil and residues contain high levels of sulfur, nitrogen, phosphorous, metals (especially vanadium and nickel); asphaltenes and exhibit a propensity to form carbon or coke on the catalyst. The sulfur and nitrogen molecules are highly refractory and aromatically stable and difficult and expensive to crack or remove. Vanadium and nickel porphyrins and other metal organic compounds are responsible for catalyst contamination and corrosion problems in the refinery. The sulfur, nitrogen, and phosphorous, must be removed because they are well-known poisons for the precious metal (platinum and palladium) catalysts utilized in the processes downstream of the atmospheric or vacuum distillation towers.
The difficulties treating atmospheric or vacuum residual streams has been known for many years and has been the subject of considerable research and investigation. Numerous residue-oil conversion processes have been developed in which the goals are same: 1) create a more valuable, preferably middle distillate range hydrocarbons; and 2) concentrate the contaminates such as sulfur, nitrogen, phosphorous, metals and asphaltenes into a form (coke, heavy coker residue, FCC slurry oil) for removal from the refinery stream. Well known and accepted practice in the refining industry is to increase the reaction severity (elevated temperature and pressure) to produce hydrocarbon products that are lighter and more purified, increase catalyst life times and remove sulfur, nitrogen, phosphorous, metals and asphaltenes from the refinery stream.
In summary, since the announcement of the MARPOL Annex VI standards reducing the global levels of sulfur in HMFO, refiners of crude oil have had modest success in their technical efforts to create a process for the production of a low sulfur substitute for high sulfur HMFO. Despite the strong governmental and economic incentives and needs of the international marine shipping industry, refiners have little economic reason to address the removal of environmental contaminates from high sulfur HMFOs. The global refining industry has been focused upon generating greater value from each barrel of oil by creating middle distillate hydrocarbons (i.e. diesel) and concentrating the environmental contaminates into increasingly lower value streams (i.e. residues) and products (petroleum coke, HMFO). Shipping companies have focused on short term solutions, such as the installation of scrubbing units, or adopting the limited use of more expensive low sulfur marine diesel and marine gas oils as a substitute for HMFO. On the open seas, most if not all major shipping companies continue to utilize the most economically viable fuel, that is HMFO. There remains a long standing and unmet need for processes and devices that remove the environmental contaminants (i.e. sulfur, nitrogen, phosphorous, metals especially vanadium and nickel) from HMFO without altering the qualities and properties that make HMFO the most economic and practical means of powering ocean going vessels.